Fatigue and Exercise: Part IV  //  Exercise in the Heat

21 May 2008 Posted by
Exercise in the heat: Predicting the physiological future – African runners outperform white runners in the heat

We’re back with Part 4 (or is it 5 or 6? I’ve lost count!) of our Series on Fatigue during exercise. In our last post, we looked at exercise in the heat, and found that:

  • Laboratory research shows that human beings will stop exercise when their body temperature rises to a certain level. That level is of course dependent on the athlete’s motivational levels, though interestingly, not necessarily on their training status or performance.
  • We also saw that the science has shown that when the body temperature rises to reach about 40 degrees, the brain actually activates less muscle than a “cooler” brain, and that there is evidence for reduced arousal and motivation.

So, the hypothesis, based on these constant workload studies, is that the heat affects performance because:

A high body temperature DIRECTLY inhibits the ability of the brain to activate muscle.
Therefore, exercise stops (because in these studies, remember, slowing down is not an option)

What happens when the athlete CAN slow down? Self-paced exercise

Today we turn our attention to the case where athletes can slow down – this is arguably more representative of what you will see in Beijing later this year, since any athlete can, at any stage, choose to drop off the pace. Of course, they lose their medal chance this way, but it’s a much more applicable form of testing.

And to understand this, we look a few studies. We’ll do it in a couple of posts, because otherwise the length would become enormous. So today, we consider one study, with more to come in the next few days.

In 2000, a study by Tatterson (J Sci Med Sport) found that cyclists slowed down soon after they started a 30-minute performance trial in hot, but not cold conditions. What was significant is that their body temperatures were not higher in the hot than in the cold when they slowed down. Obvious, yes, but quite contrary to the theory that your brain stops activating muscle AFTER your body temperature hits the “threshold”. They didn’t measure any index of muscle activation, however, but it was a crucial observation that something else (and not direct body temperature) was playing a role in the heat.

African runners in the heat – anticipatory regulation thanks to their smaller size?

Then, a study done by Frank Marino while he visited Cape Town a few years back, was one of the first to use the words “anticipatory”, because his finding (discussed below) found differences in the pacing strategy of African runners compared to white runners in hot conditions. So the conclusion is that something is happening BEFORE the body temperature rises, slowing the runner down so that they don’t overheat.

And this is obvious. Think for a moment about when you go and train on a very hot day. You do not simply go out and run or cycle at your normal pace until suddenly, overcome with a sensation of hyperthermia, you slow down! Rather, your entire approach to the session is changed and you slow down LONG BEFORE you ever get hot in the first place! Within the first few strides, you’re probably already going slower. So this is one of those examples we spoke about a long time ago – intuitively, we know what happens.

The question is HOW? And also, we have to consider the prevailing expert opinion of the time. In this case, remember, the “textbook” knowledge says that exercise is impaired because the HOT BRAIN directly inhibits muscle activation after body temperatures are raised by exercise.

So, let’s look at the study by Frank Marino. I’m sure he’ll forgive my very rudimentary depiction of his methods below:

Marino-methods

So he had 6 African and 6 white runners, quite well trained, doing a performance trial after a 30 minute steady run in either HOT (35 degree) or COOL (15 degree) conditions.

The starting hypotheses for this study, had you read the theories about exericse in the heat, would be:

  • Performance would be impaired in the heat, so the runners would be slower during the 8km trial in the hot condition. This is fairly obvious.
  • They’d slow down in the HOT trial because they’d be much hotter than in the cool trial – the high body temperature (and HOT brain) is failing to activate muscle, as we’re told by other research.

This is what was found:

Graph of running speed (km/hr) against time for the 12 runners during 8km time-trial performances preceded by 30minutes run in hot and cold conditions

Graph of running speed (km/hr) against time for the 12 runners during 8km time-trial performances preceded by 30minutes run in hot and cold conditions

I’ve highlighted with a red circle one of the more significant findings – the white runners started the 8km trial much slower than the black runners did, from the first minute. Of course, both groups eventually slowed down in the heat compared to the cold (the black symbols on the graph), but it’s this difference between black and white runners that should be of interest. So, why then, do the white runners start so much more slowly?

Option 1 is that they are already hot. They might be finishing the 30 minute steady run with higher body temperatures. That would agree with the theory that the hotter you are, the slower you go…

However, look at the graph below:

Graph of rectal temperatures during the course of the trials in hot and cold conditions

Graph of rectal temperatures during the course of the trials in hot and cold conditions

Again, I’ve highlighted the key point there – the black and white runners had THE SAME rectal temperature when they started the 8km run. And not only this, but the temperature was “only” 38.2 degrees, so they were way cooler than the supposed “limiting temperature”.

Yet, for some reason, despite the fact that the black and white runners have the same temperature and are not in any danger, the white runners “chose” to START an 8km time-trial slower than the black runners. We can therefore dismiss Option 1 from above, and say that it’s clearly not a case of a hot athlete slowing down! If it was this simple, with some “direct effect” on the athlete, then the slowing down would happen equally in the two groups. This is an amazing finding given the prevailing view that the heat impairs performance directly, I hope it strikes you that way too!

So what, then, is the reason? Well, that’s of course difficult, if not impossible to PROVE (as we’ve seen recently courtesy the CAS, “proof” in science is not as easy to do as people think), but here’s a theory from the Marino paper:

  • The African runners were much smaller than the White runners – 59 kg compared to 77kg, to be exact. The white runners were taller, however, and had a larger body surface area.
  • We know from previous research that a smaller runner produces less heat while running at the same speed as a larger one. That is, the total heat PRODUCTION is dependent on body mass, and smaller people produce less heat.
  • Smaller runners also lose less heat, however, because they have a smaller body surface area to lose heat to environment.
  • But the key is: These two factors don’t exactly cancel one another out. The result is that even though they lose less heat, smaller runners are still able to lose more heat RELATIVE to heat production than larger runners. This has to do with the ratio their mass to body surface area – they may lose on surface area, but their lighter weight more than makes up for it.
  • The net result of all this, is that smaller athletes have a reduced RATE OF HEAT STORAGE than bigger runners.
  • Now, given this fact, if two runners are going along at the same speed, the smaller one will be storing less heat, and therefore his/her body temperature will be climbing slower than that of the big runner.
  • Put differently, it means that if both athletes are concerned about how hot they are getting, then the bigger runner will have to slow down in order to prevent his heat storage from rising, which would ultimately increase his heat production.

Now, with all those facts on the table, the results start to offer an interesting theory:

The rate of heat storage is responsible for Anticipatory Regulation of exercise and pacing strategy in the heat

The theory is that the white runners, by virtue of their bigger size, have an increased rate of heat storage. (Note that this effect (the different pacing strategies, that is) is likely due to size – had the groups been matched for mass and height, the result might have been different – see the comments section to this post!)

The brain is “clever” enough to know that if the athletes starts their 8km time-trial at a fast pace, then their very high rate of heat storage is going to see their body temperature RISE very rapidly. They are in danger of reaching a core temperature of 40 degrees BEFORE the end of the time-trial (which they know is 8km long). Remember, at this temperature, the brain says “Enough” and exhaustion usually occurs (or soon after).

Therefore, the brain says “Whoa, back off a little!”, long before the athlete overheats, and with the intention of making sure that they do not reach this limiting temperature before they are able to finish the trial – it would be a complete failure to do this, and reach the 6km mark by the time their brain says “enough”. So instead, it REGULATES their performance IN ANTICIPATION of ever reaching that limit. That Anticipatory Regulation is achieved or mediated by the rate of heat storage, which is different from the very early stages of exercise.

On the other hand, the African runners, who are smaller, have no such problems. They thus maintain a higher speed, and a similar rate of heat storage, leading ultimately to an improved performance. Note, very importantly, that in the cold, this difference between black and white runners does not exist. Therefore, it’s not a case that the white runners are just inferior to the black runners – it applies only in the heat, when the environmental temperatures bring this heat storage aspect into play.

Looking ahead

What this study does not do is measure anything related to brain function. Now, that’s very difficult to do during dynamic exercise, and is often criticized, but we’ll discuss a study tomorrow that looked at EMG activity (a measure of how much muscle is being activated by the brain) during trials in the hot and cold. This was the first study to find evidence for it. It was also a study I did for part of my PhD, though I’m not claiming anything here – it was be default, more than anything else!

So that’s coming up in our next post – evidence of Anticipatory Regulation of Exercise Performance, along with a few more concepts to build on the ideas put forward here.

Join us then!

Ross